1. Field of the Invention
The present invention relates to an oscillator and a communication apparatus including the same. More particularly, the present invention relates to an oscillator having a resistance changing unit which changes the magnitude of output signals, and also relates to a communication apparatus including the same.
2. Description of the Related Art
An oscillator, particularly, a voltage control oscillator capable of controlling an oscillation frequency with a voltage, is used as a local transmission source in a receiver or an intermediate-frequency signal source in a transmitter. In a communication apparatus having both a receiving function and a transmitting function, a single voltage control oscillator is commonly used by the receiver and the transmitter.
FIG. 6 is a block diagram of a voltage control oscillator module 50 according to the related art. The voltage control oscillator module 50 includes a voltage control oscillator 53 having an oscillator circuit 51 and an amplifier circuit 52 which amplifies an oscillation signal of the oscillator circuit 51, an attenuator circuit 54 for attenuating an output signal S1 from the voltage control oscillator 53 in response to a control signal, a control terminal 55 from which a control voltage is applied to the oscillator circuit 51, a power supply terminal 56 of the oscillator circuit 51 and the amplifier circuit 52, and an output terminal 57 through which a local oscillation signal is output.
The attenuator circuit 54 operates to pass or to attenuate the output signal S1 of the voltage control oscillator 53 by switching a switch SW51. Specifically, when a communication apparatus such as a cellular telephone system which carries the voltage control oscillator 53 does not work, the magnitude of an output signal So from the output terminal 57 is reduced to a predetermined value or less. This prevents the output signal So from leaking when the cellular telephone system is not working.
More specifically, the attenuator circuit 54 shown in FIG. 6 is a T-shaped attenuator including two resistors R51 and R52 that are connected in series to the output from the voltage control oscillator 53, and a resistor R53 connected between a node of the resistors R51 and R52 and the ground. The attenuator circuit 53 allows the amount of attenuation applied to the output signal S1 from the voltage control oscillator 53 to be set according to the values of resistors R51 to R53.
However, since the voltage control oscillator module 50 uses the external attenuation circuit 54 to attenuate the output signal S1 from the voltage control oscillator 53, the output signal S1 from the voltage control oscillator 53 must always be maximum. As a result, the current flowing to the voltage control oscillator 53 must be maximum, leading to a problem that the lifetime of the battery incorporated in the communication apparatus is severely reduced.
In order to overcome the problems described above, preferred embodiments of the present invention provide a voltage control oscillator that reduces current flow when an apparatus incorporating the voltage control oscillator does not work, and also provide a communication apparatus including such a voltage control oscillator.
According to one preferred embodiment of the present invention, an oscillator includes an oscillator circuit, and an amplifier circuit for amplifying an oscillation signal from the oscillator circuit, the oscillator circuit and the amplifier circuit being connected to each other. The amplifier circuit includes an amplifier transistor, and a resistance changing unit between the emitter of the amplifier transistor and the ground.
The oscillator circuit may oscillate while switching between a plurality of oscillation frequencies, and the resistance changing unit may be switched according to the oscillation frequencies of the oscillator circuit.
The resistance changing unit preferably includes a resistor and a switch.
In another preferred embodiment of the present invention, a communication apparatus includes at least one oscillator according to the preferred embodiment described above.
In an oscillator according to a preferred embodiment of the present invention, a resistance changing unit is preferably provided between the emitter of an amplifier transistor contained in an amplifier and the ground, thereby changing a current flowing between the emitter of the amplifier transistor and the ground.
A communication apparatus according to another preferred embodiment of the present invention includes at least one oscillator that reduces current flow, thereby extending the lifetime of the battery incorporated therein.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.